Welcome to the world of WordPress.

When you upload an image to your media library, your WordPress installation will automatically make some smaller versions of the same image and link this all together. So when you later select that you want to include a small copy of an image, then the smaller file will be used.

BUT: The list of what sized should be prepared is dynamic. Most of all, the list of sizes depends on which theme you’re using.
So I suggest that you first start with a small number of images and play around, until you’ve found a theme you like and your set uf plugins seems to do their work ok. And THEN you start uploading the bulk.
Prepare your images for the largest image size you believe you’re going to need.
Have in mind that retina displays can be very pixel-hungry. HighRes will look beatiful, but it’s going to cost bandwidth, load time and storage.

Through some special tweaking, it’s possible to adjust the list of standard sizes. That may come in handy, should you need a particular size of image, say a thumbnail that isn’t square…

Good luck!

Note that using Photoshop to compress files before uploading them is a great practice for image optimization. You can save a lot of file space and bandwidth by optimizing them in Photoshop.